1. Field of the Invention
The present invention relates to an integration method and integration circuit widely adopted for electronic circuits, and a voltage-controlled oscillator adopted widely for electronic circuits and to a frequency-voltage converter employing the integration circuit. More particularly, this invention is concerned with an integration method and integration circuit suitable for applications in which a variation in a factor relevant to charging or discharging an integrating capacitor falls within a predetermined narrow range. This invention is also concerned with a voltage-controlled oscillator and frequency-voltage converter employing the integration circuit.
2. Description of the Related Art
An integration circuit is widely adopted as part of an electronic circuit. When supply of a direct current to an integrating capacitor is started, a terminal voltage or a voltage developed at a terminal of the integration circuit rises with the passage of time. Assuming that a current is I, the terminal voltage is Vc, the capacitance of the integrating capacitor is C, and an integration time is T, the relationship of Vc=(I/C).times.T is established. Herein, C is a fixed value.
When the integration circuit is used as part of an electronic circuit, the above relationship is utilized. One of I, T, and Vc is set to a predetermined value, and another one thereof is varied. A variation in the remaining one is detected. For example, the integration circuit may be used as part of a voltage-controlled oscillator. In this case, two reference voltages are determined, and the current I is varied proportionally to a voltage for inducing charge. When Vc reaches a first (higher) reference voltage, an output signal is varied. At the same time, the current I is varied for inducing discharge. When Vc reaches the second (lower) reference voltage, the output signal is varied. The current I is varied for inducing charge again. Thus, the output signal oscillates at a frequency dependent on a voltage.
In this kind of voltage-controlled oscillator, for example, a noise may be superposed on a reference voltage. In this case, a discharge period during which an integrating capacitor is discharged becomes unstable. This causes jitter in the output signal, and poses a problem in that a high-precision oscillating signal cannot be produced. As long as the magnitude of the noise is unchanged, the jitter relates to the ratio of the current I to the capacitance C, I/C. To reduce the jitter, the ratio I/C must be raised, and a change rate at which the terminal voltage of the integral capacitance makes a transition must be raised. However, the integration time is determined with the cycle of the oscillating signal. A difference between the two reference voltages cannot be increased. Consequently, it is impossible to raise the ratio I/C.
Moreover, a frequency-voltage converter employs the integration circuit as part of an electronic circuit. An input signal is, for example, a zero-crossing signal. When the input signal crosses a zero level, a pulsating signal is generated. The pulsating signal is used to reset the integration circuit. A terminal voltage or a voltage developed at a terminal of the integration circuit is sampled and held. An output signal is then produced based on the terminal voltage.
In the frequency-voltage converter, if a change rate at which the frequency of an input signal changes is low, a change rate at which the voltage level of an output signal changes is also low. This poses a problem in that the output signal is susceptible to a noise and the frequency-voltage converter suffers from a poor signal-to-noise ratio in terms of the output signal. To solve this problem, the ratio I/C of the current I to the capacitance C should be increased, and the change rate at which the terminal voltage Vc of the integrating capacitor makes a transition should be raised. When I/C is raised, Vc exceeds a voltage level that can be sampled and held. This is a problem and I/C cannot therefore be raised.
As mentioned above, as far as a circuit employing the integration circuit is concerned, it is preferred to raise the ratio I/C of a current to a capacitance for minimizing the influence of a noise. In practice, however, there is difficulty in raising I/C.